This invention relates to an annular fluid composition for insulating a wellbore. More particularly, this invention relates to an annular fluid composition that may be economically prepared and is environmentally acceptable.
The annular fluid composition of this invention is particularly well suited for use as a wellbore insulation material in subsea hydrocarbonaceous fluid drilling operations. Throughout the specification, numerous references will be made to the use of the fluid in subsea drilling operations, however, it should be realized that the invention could be used in any drilling operation which requires an annular fluid and, in fact, in any application requiring a thixotropic insulating fluid.
Annular fluids or packer fluids are liquids which are pumped into an annular opening between a casing and a wellbore wall or between adjacent, concentric strings of pipe extending into a wellbore. These fluids are especially necessary in drilling operations conducted in low temperature venues of the world, for example, those areas having a permafrost. A permafrost is a thick layer of frozen surface ground which may be several hundred feet thick and presents a great obstacle to the removal of relatively warm fluids through a well pipe. Particularly, the warm fluid in the well pipe causes thawing of the permafrost in the vicinity of the well resulting in subsidence which can impose compressive and/or tension loads high enough to fracture the wall casing and hence allow the escape of oil. In addition, the warm gas or oil coming to the surface in the well pipe becomes cooled by giving up its heat to the permafrost and hydrate crystals are formed which can freeze together and block the well pipe.
Heavy oil production is another drilling operation which often requires a packing fluid. In heavy oil production, a high pressure steam is injected into the well and the oil reservoir to heat the fluids in the reservoir, causing a thermal expansion of the crude oil, an increase in reservoir pressure and a decrease of the oil's viscosity. In this process, damage to the well casing may occur when heat is transferred through the annulus between the well tubing and the casing. The resulting thermal expansion of the casing can break the bond between the casing and the surrounding cement, causing steam leakage between the casing and the oil well wall. Accordingly, an insulating medium such as a packer fluid is used to insulate the well tubing. The packer fluid also reduces heat loss and saves on the energy requirements in steam flooding.
In addition to steam injection processes and operations which require production through a permafrost layer, subsea fields required specially designed systems which typically require a packer fluid. For example, a subsea oil reservoir temperature may be between about 120.degree. F. and 250.degree. F., while the temperature of the water through which the oil must be conveyed is often as low as 32.degree. F. to 50.degree. F. Conveying the high temperature oil through such a low temperature environment can result in an oil temperature reduction and consequently the separation of the oils various hydrocarbon fractions and deposition of paraffins, waxes, asphaltenes, and gas hydrates. The agglomeration of these oil constituents can cause blocking or restriction of the wellbore, resulting in significant reduction or even catastrophic failure of the production operation.
To meet the above-discussed insulating demands, a variety of packer fluids have been developed. For example, U.S. Pat. No. 3,613,792 describes an early method of insulating wellbores. In the 3,613,792 patent, simple fluids and solids are used as the insulating medium. U.S. Pat. No. 4,258,791 improves on these insulating materials by disclosing an oleaginous liquid such as topped crude oils, gas oils, kerosene, diesel fluids, heavy alkylates, fractions of heavy alkylates and the like in combination with an aqueous phase, lime, and a polymeric material. U.S. Pat. No. 4,528,104 teaches a packer fluid comprised of an oleaginous liquid such as diesel oil, kerosene, fuel oil, lubricating oil fractions, heavy naphtha and the like in combination with an organophillic clay gellant and a clay dispersant such as a polar organic compound and a polyfunctional amino silane. U.S. Pat. No. 4,877,542 teaches a thermal insulator fluid consisting of a heavy mineral oil as the major liquid portion, a light oil as a minor liquid portion, a smectite-type clay, calcium oxide and hydrated amorphous sodium silicate. U.S. Pat. No. 5,290,768 teaches a thixotropic composition containing ethylene glycol and welan gum. The above-discussed patents are herein incorporated by reference.
Although many of the above-described packer fluids function adequately, they fail to meet the ever increasing industrial and governmental concerns for the environment. Particularly, many of the constituents of the above-described packer fluids are unacceptable from an environmental standpoint and are often prohibited for use by government regulation. For example, the mineral oils and heavy crude oils required by several of the above discussed patents are not permitted for use in areas such as the Gulf of Mexico.
Among the above patents, U.S. Pat. Nos. 3,613,792 and 5,290,768 describe insulating fluids which may be environmentally acceptable. However, U.S. Pat. No. 3,613,792 describes relatively ineffectual insulating fluids which are clearly inferior to the thixotropic agents of the present invention. U.S. Pat. No. 5,290,768 describes a allegedly environmentally acceptable packer fluid, however, the packer fluid contains ethylene glycol which is not readily biodegradable and may be corrosive.
Accordingly, a significant advantage is provided to the art by the present annular fluid having excellent insulation properties and environmental acceptability.